1. Field to which invention relates
The invention relates to an expanding masonry attachment plug, preferably of plastics material, whose plug head has a wide insertion hole and whose shank, provided with a through hole, is divided by longitudinal slits or slots into expanding segments, which join at the far end to form a plug foot.
2. The prior art
Expanding masonry attachment plugs of a conventional type are so constructed that on insertion of the screw by twisting into the central guide hole of the shank the expanding segments, starting at the plug foot, expand or spread out and under the expanding pressure of the conically shaped wood screw come to lie against the wall of the hole made for the plug. In this classical form of expanding plug the expanding elements are only held together by means of narrow connecting pieces of the plug foot, the connecting pieces being intended to prevent a premature divergence of the segments on hammering the plug into a plug hole and consequent prodding against the wall of the hole. This prior art is represented for example by the British patent specification No. 589,648 and the Swiss patent specification No. 212.
In addition to these expanding masonry attachment plugs expanding plugs have recently been proposed in the case of which the expansion or spreading out of the segments is not brought about internally by driving home the screw with a screw driver and instead such an expansion is brought about by an axial stressing of the shank by screwing the screw into a nut arranged at the foot of the plug, the screw which is being driven home cutting its own female thread in a closed sleeve at the foot of the plug.
Masonry attachment plugs of the last-mentioned type have been proposed owing to the use on an increasing scale of constructional materials which have a low resistance or strength. Such materials include more particularly gas concrete, pumice-like materials or hollow building blocks. In the case of the use of conventional expanding masonry attachment plugs in such light-weight building blocks the high radial expanding forces cause a collapse of the cells adjacent to the wall of the hole so that there is no sufficiently high resistance opposing the expanding forces and secure attachment becomes impossible. The bending effect of the latter-mentioned type of expanding plug however leads to a very substantial expansion outwards and even in the case of spreading out within a cavity it is possible to ensure that the attachment cannot be pulled out again, because the bent expanding connecting pieces form an abutment, which catches behind the hole and acts as a source of blind rivet.
Masonry attachment plugs of the latter-mentioned type are described for example by the German Gebrauchsmuster Specification Nos. 1,860,318 and 1,935,793, the Swiss patent specification No. 486,644, the French patent specification No. 1,278,641 and the German patent specification (Offenlegungsschrift) No. 2,060,241.
However, such masonry attachment plugs operating with an axial clamping or stressing effect are not suitable for work in concrete and other solid or dense constructional materials because the radial spreading force can only be relatively small as a consequence of the axial stressing taking place in a hole properly dimensioned to suit the plug, for this force must be transmitted by the expanding elements themselves, which surround the screw which has been placed in position, in the manner of a bridge. Furthermore, in the case of an excessively pronounced axial stressing a destruction of the screw thread in the plug foot is to be feared.